The term definitely predates D&D - the term "twenty dollar gold piece" has been in use for the $20 Double Eagle and $10 Eagle coins of the late 19th century, and also the $5 gold coin, as well.
"Gold Piece" In Print
The term is used in the Lebanon Daily News, 1 Nov 1965, of Lebanon, Pennsylvania, bottom, in an advert for old coins under the left column of text (to the right of the comics)
Four gold pieces: One (1) $20.00 gold piece, two (2) $10.00 gold pieces and one (1) $2.50 gold piece.
This alone establishes the phrase "gold piece" for gold coins in routine use prior to D&D. But let us press a little further back... say, 1913? Here's a quote from the 5 August 1913 Cedar Rapids Evening Gazette, from Cedar Rapids, Iowa, page 4, top of the third column:
The five cent piece ls the day laborer of our coinage. It la the hardest working and most successful bit at money In use In these United States. The twenty dollar gold piece Is very popular and is madly sought after In the best society; the five dollar bill has millions of friends and the hard silver dollar can be found nestling In the pocket of almost every man. But none of these like the five cent piece.
We thus have established a pattern of use for gold coins of being called "gold pieces" in the press, spanning over 5 decades; clearly not a D&D origin; not even viably a wargaming origin, for 1913 is the year of the first printing of H. G. Wells' Little Wars, the first commercially released set of wargaming rules in book form.
Searching Project Gutenberg, several ebooks have it in use...
These without clear denomination prefixed:
- Pinocchio (1883, Tr. ??? )
Author: Carlo Collodi, 1826-1890
Translator: Carol Della Chiesa, 1887-
- The Younger Set (1907)
Author: Robert W. Chambers
- A Drama on the Seashore
Author: Honore de Balzac (1799-1850)
Translator: Katharine Prescott Wormeley (1830-1908)
- Tiger Cat (1938)
Author: David H. Keller
- Pâkia (1901)
Author: Louis Becke
And several with clear denomination in dollars:
Piece
Piece is, according to several dictionaries, a common term for coins in general, of whatever denomination is specified. The quote below is excerpted from the etymology online page:
piece
early 13c., "fixed amount, measure, portion," from O.Fr. piece (11c.), from V.L. *pettia, probably from Gaulish (cf. Welsh peth "thing," Breton pez "piece"), from O.Celt. base *pett-.
[...]
Piece of Eight is the old name for the Spanish dollar (c.1600) of the value of 8 reals.
Commentary
It's pretty clear that it's a generic term for a gold coin, and for several US gold coins as well. In the US, it seems to be predominantly the popular $5 coin of the late 19th and early 20th centuries, but can be used collectively for the $2.50, $5.00, $10.00 and $20.00 gold coins; The silver coins of similar values were $0.10, $0.25, $0.50, and $1.00. Note that, still to date, "2 bits" is $0.25... a reference to the not uncommon practice of breaking Pieces of Eight (Dollares, or Reals) into 8 "bits" of an eighth-dollare each... I suspect that this is the origin of the 20:1 Silver:Gold ratio in AD&D...
That's interesting, as being a non-native English speaker I always assumed it was one of the accepted meanings. So as every time I realize one of these things, let's check the Wiktionary entry for soak:
Verb
soak (third-person singular simple present soaks, present
participle soaking, simple past and past participle soaked)
- (transitive) To allow (especially a liquid) to be absorbed; to take
in, receive. (usually + up)
I soaked up all the knowledge I could at university.
So yes, even though it is not the most often used meaning, "to soak" can be used for "to absorb", and not only for liquids. So "soaking damage" for "receiving damage and absorbing it" is not a pure invention, it is accepted - though unusual - English language.
Best Answer
Breastplate and half-plate were added in late AD&D 2e to represent Renaissance-era lighter plate armor.
The summary is as follows:
Usage in D&D
Chainmail (1971) featured plate armor among its many armor types, but no breastplate or half-plate. Similarly, the original D&D boxed set's Men & Magic (1974) again featured plate armor as the strongest, but no breastplate or half-plate.
The AD&D 1e Player's Handbook (1978) introduced the term "plate mail" for its most superior armor. The Dungeon Master's Guide (1979) p.27, describes this as light chainmail with some plates, and also briefly mentions plate armor and field plate. On DMG p.165, Gygax cites Charles Ffoulkes' 1909 work Armor and Weapons as a source.
Half-plate and breastplate still do not appear in the AD&D 1e core rulebooks. As far as I can find, half-plate did not appear in any AD&D 1st edition sourcebook.
The term "breastplate" did appear in various sourcebooks, often describing a component of a more complete suit of armor. There are some instances of people wearing a breastplate alone, usually primitive tribes such as the Rovers of the Barrens (World of Greyhawk box set) or the people of Hyboria (Conan sourcebooks), and usually made of some inferior material. We do see a thief in Lankhmar sourcebook Swords of the Undercity wearing a breast plate alone.
Unearthed Arcana makes major changes to plate-type armor, and defines more clearly what each represents. Plate mail is chain or brigandine with some plates over vital areas. One point better is the new "field plate" armor, and one point better again is the "full plate" armor representing late middle ages and renaissance plates. AD&D 2e's Player's Handbook adopts these new armor types as standard.
AD&D 2e's Player's Option: Combat & Tactics (1995) introduces "half-plate" to represent a chronologically later form of plate armor, from a time when firearms became more prevalent, reducing the benefit of full-body armor. It covers the chest, outer arms, and upper legs. Historically, lower-leg armor was disadvantageous for soldiers who marched on foot, as it significantly increases the amount of energy used when marching.
Combat & Tactics also has a type of armor called "back-and-breast", which is a metal breastplate and backplate, based on armor worn during the renaissance. Historically, armor like this exists which was able to protect against a shot to the chest from early firearms, which was the most important risk to mitigate as firearms came to dominate the battlefield. Additional armor was an impractical expense and impediment to movement.
In D&D 3e's Player's Handbook, the armor list features breastplate, half-plate, and full plate. These are likely drawing on Combat & Tactics, which shares lead designer Skip Williams and was written only a few years earlier.
While D&D 4e's Player's Handbook ditched breastplate and half-plate, that edition was generally poorly received, and D&D 5e draws more from 3e. D&D 5e thus uses breastplate, half-plate and full plate. Of note, 5e recategorizes half-plate from heavy to medium armor; in 3e it was all-round inferior to full plate and never used, whereas now it's the highest-AC medium armor.
History of terminology
The word "breastplate" appears in the King James Bible, published in 1611. Revelation 9:9 says:
This book dated 1895 also uses it to refer to armor, suggesting (perhaps through Biblical use) it was still a word in common use at least among academics:
Breastplate explicitly referencing a metal piece of chest armor is similarly used in Andrew Lang's 1906 work Homer and His Age, an article which also quotes 1800s works on the armor worn by the Iroquois. It uses the word to describe Greek armor:
I can't find any similar historic reference to "half-plate" before 1899, so I'm skeptical that it was a term used at the time this armor was in use. However, it does pre-date D&D. Compton's Pictured Encyclopedia (1922) p.215, describes both "full plate armor" of the 16th century, and "half-plate" worn by 14th century mounted knights.
The term "full plate" appears in Charles Ffoulkes' 1909 work Armour and Weapons, which Gygax cites in the 1e DMG as a source.